This invention relates to a process and apparatus for delivering a liquid cryogen to a use point. More particularly, the invention relates to a process and apparatus for delivering small quantities of a liquid cryogen, such as liquid nitrogen, liquid oxygen, liquid argon, etc., intermittently to a use point in a controlled manner.
Many applications, such as cooling of dies in aluminum die extrusion, purging of cans in the canning industry, etc., require supplying relatively small (up to about 400 lbs/hr) controlled amounts of a cryogenic liquid, e.g. liquid nitrogen, to a use point. In such applications, control of the amount of liquid dispensed, control of the amount of refrigeration provided, and control of the timing of dispensation are very important. In transferring the liquid cryogen from a (remote) supply source to the use point, the conventional practice of transferring liquid through an insulated conduit is often unacceptable because of heat leaks which cause a relatively large fraction of the liquid mass transferred to vaporize, especially in applications such as aluminum die extrusion which involve high temperatures at or near the use point. For example, in a typical aluminum die extrusion plant, a 3/4 in. nominal diameter liquid nitrogen pipe, insulated with 3 in. of urethane foam, will vaporize, because of heat leak alone, about 13.9 lbs/hr of liquid nitrogen for every 150 ft. of a pipe length, i.e. an amount at least within the same order of magnitude as the amount of the desired liquid nitrogen flow used in cooling the aluminum extrusion die, and often many times that amount. This causes problems with cryogen flow control and interferes with control of the amount of liquid cryogen delivered to the use point, resulting in loss of refrigeration at the use point and/or waste of liquid cryogen.
In addition, if the flow of liquid cryogen is intermittent, the problem is compounded further because the residual liquid in the pipe continues to vaporize during non-use periods, necessitating purging the pipe of vapor before any liquid can be delivered to the use point, thus slowing down the process and often resulting in further waste of cryogen.
Furthermore, heat leak is not the only factor causing vaporization. An additional, though usually smaller, amount of cryogen vaporizes by flash vaporization due to pipe line pressure drop. This flash vaporization factor may become very significant, especially when cryogen is transferred from a remote supply source and undergoes a change in elevation.
It is desirable, therefore, to have a liquid cryogen delivery system which is capable of delivering small predetermined quantities of a liquid cryogen (i.e. essentially free of vapor) promptly to a use point in a controlled manner and on an intermittent basis, and with minimum loss of refrigeration.
One technique used in the prior art for transporting liquid cryogen has involved a concentric pipe system having a coolant in the outer jacket. Concentric pipe construction, however, is expensive, complicated, not suited to intermittent flow, and inefficient because vaporization of the jacket liquid results in coolant waste. Use has also been made of phase separators designed to separate and vent vaporized cryogen. Such separators, however, deliver liquid in a saturated liquid state (at line pressure) and fail to prevent further vaporization downstream.